The conventional RF-MEMS switch is a mechanical switch having movable members in a membrane or rod form supported at both ends or in a cantilever, so that by placing them into or out of contact with the electrodes, signal propagation path can be switched over. Although the power sources for driving the membrane or movable member, in many cases, use those based on electrostatic force, there are released ones using magnetic force.
As a micro-fabricated switch in a size around 100 μm, there is known one described in IEEE Microwave and Wireless Components letters, Vol. 11, No 8, August 2001 p334. This switch forms a signal line for radio-signal transmission over a membrane, to provide a control electrode immediately beneath the signal line. In case a direct current potential is applied to the control electrode, the membrane is pulled and deformed toward the control electrode by an electrostatic force. By a contact with a ground electrode formed on the substrate, the signal line formed on the membrane becomes a shorted state. Due to this, the signal flowing through the signal line is attenuated down or blocked off.
Unless a direct current potential is applied to the control electrode, there is no deformation in the membrane. The signal flowing through the signal line on the membrane is allowed to pass through the switch without encountering the loss through the ground electrode.
Meanwhile, as a conventional method for controlling the positioning of the movable member, there is known an art shown in JP-A-2-7014. This structure is arranged to open and close an optical path by a micro-switch, thereby turning the signal on/off. When to pass light, a voltage is applied to between a vibration plate and a flat plate, to lift the element through an electrostatic force. When to block light, voltage is rendered zero to cancel the electrostatic force whereby it is returned to the former position by a spring force of the vibration plate. Due to this, the element blocks the light.
At this time, in case the voltage is abruptly applied or reduced to zero, a phenomenon called chattering takes place, resulting in vibration of the element. It takes a time in reaching a stability. Consequently, it is a practice to apply a voltage called a preparatory voltage pulse before applying a control voltage, thereby preventing chattering. The condition for stabilization is determined by a preparatory pulse voltage V1 and a pulse width τ1, and a spacing τ2 between the preparatory pulse voltage V2 and the major control voltage. In case V1=V2 and τ1=τ2 is assumed, then τ1 has a boundary condition of one-sixth of the eigenfrequency.
The research and development of RF-MEMS switch in the IEEE Microwave and Wireless Components letters originates aiming at those for the military and aerospace applications, wherein the research and development is focused on by what means signal propagation characteristic is to be improved. However, in the case of the home-use application including personal digital assistants, there is a desire for an RF-MEMS switch meeting simultaneously various conditions of durability, high-speed response, low consumption power, low driving voltage, size reduction and the like, besides improved signal propagation characteristic as a natural matter.
However, the direct current voltage of as high as approximately 30 V or more is required to contact the membrane toward the control electrode. It is not preferred to build such a switch as needing a high voltage within a radio transceiver apparatus.
Meanwhile, in order to achieve high electrical isolation on a switch, it is required to provide a comparatively wide gap between a movable member and an electrode. In such a case, it is critical by what means the movable member is to be driven with a great displacement and high speed on a low drive voltage.
Also, on the RF-MEMS switch for example, when the movable member is contacted on the electrode, in case the drive voltage is turned off into a state not to give an electrostatic force to the movable member, the movable member is returned by its own spring force to a predetermined position distant from the electrode. For contacting the movable member at high speed to the electrode by a low drive voltage, the spring force of the movable member must be weakened. This, however, poses a problem of low response speed for the movable member to return to a predetermined position.
Also, on a mechanical switch, in returning the movable member contacted with the electrode to a position where isolation is high not to cause a capacitance coupling of movable member and electrode, there is a problem of overshoot, i.e. the movable member is to displace beyond the predetermined position. Where the overshoot of movable member is great, capacitance coupling possibly takes place on the electrode and movable member, as a signal propagation path, resulting in forming an incorrect signal path.
On the other hand, the switch of JP-A-2-7014 requires a sufficient connection area in order to secure a capacitance during switch-on. In the case the beam assumably has a width of several μm, the beam has a length on the order of several hundred μm. Accordingly, it is difficult to fix a beam having a length of several hundred μm only at one end. Higher stability is available rather by a both-ends-supported beam fixed at both ends.
However, where fixed at both ends, the substrate and beam materials, if different, cause a change of internal stress due to a difference in the thermal expansion coefficient between the materials, thereby changing the spring constant. The eigenfrequency of a structural body is determined by a mass and spring constant of the beam, as shown in Equation 1. Accordingly, temperature change causes eigenfrequency change correspondingly.f=l/π√{square root over (k)}/m   Eq.1 
Even in case a preparatory pulse voltage is applied to avoid chattering, a switch temperature change causes a change of eigenfrequency, hence changing the optimal preparatory pulse voltage. For example, when the preparatory pulse voltage is optimized at room temperature, a rise in switch temperature causes an eigenfrequency increase. Based on a preparatory pulse voltage same as that at room temperature, it is impossible to prevent chattering.
From these problems and requests, there is a desire for a switch realized with switch high-speed response on low driving voltage and a widened gap at between the movable member and the electrode, enabling to increase the response speed for the movable member contacted on the electrode to return to a predetermined position distant from the electrode and to control the magnitude of an overshoot of the movable member.